1. Field of the Invention
This invention relates to a piston and, more particularly, to a low mass piston adapted for use in pressurizing and expelling material from an aerosol container.
2. Description of Related Art
Low mass piston systems for use in aerosol containers in which the pressurizing gas is isolated from the material to be dispensed are well known. One application for which such systems are particularly well suited and in wide use is for post-foaming shaving gel products. Examples of such piston and can arrangements suitable for this and other applications are described in, for example, U.S. Pat. Nos. 4,703,875 and 4,913,323. Both of these patents describe pistons the largest outer diameter of which is slightly smaller than the inside diameter of the can such that a film of the material to be dispensed creates a seal between the piston and the can to prevent the pressurizing gas beneath the piston from bypassing the piston and mixing with the material to be dispensed. It will be appreciated that this arrangement, by calling for a typical film clearance of at least a few thousandths of an inch in the case of post-foaming shaving gel, results in a looseness of the piston in a can before either the material or the pressurizing gas are injected into the can. This looseness can result in a tipping or canting of the piston as the can is handled following assembly. This tendency to tip is enhanced in the case of stepped sidewall pistons of the types described in the two referenced patents. Such tipping, which causes a nonuniformity in the gap between the lower edge skirt of the piston and the sidewall of the can, can result in malfunctions in operation, including the escape of the pressurizing gas into the material to be dispensed.
Certain can configurations can aggravate the instability of the piston and its tendency to tip either before or during the filling operation. One such can configuration is one in which the cylindrical sidewall is necked in for the lower few millimeters of the can to enhance the appearance of the can. It has been found that the sloping transition zone in the can where the sidewall necks down to the smaller diameter, lower portion of the can, coupled with the flexibility of the lower edge of the piston, provides instability and allows considerable movement and tipping of the piston, which can lead to the problems noted above.
A primary object of the present invention is to provide a low mass piston system with improved piston stability, including when used in conjunction with necked in cans.
This and other objects will be apparent from the following summary of the invention and the detailed description of the preferred embodiment.